The coagulation of human blood is vitally dependent on the circulating blood platelet. Platelets adhere to tissue at the injury site (adhesion); they become stickly and clump to help plug the wound (aggregation); and they consolidate the clot by pulling in fibrin strands and squeezing out excess liquid (clot-retraction). All three platelet functions are dependent on the platelet membrane interacting with another macromolecule(s). The objective of the proposed research is to understand the nature of the platelet membrane-fibrin interaction necessary for clot retarction. Assay procedures for platelet-fibrin binding have been developed and will be used to further characterize the interaction. Attempts will be made to solubilize and isolate the fibrin interaction site (receptor) located on the platelet membrane. Chemical and enzymatic crosslinking procedures will be used to crosslink the platelet-fibrin complex. The crosslinked complex will be characterized. Techniques will be developed, utilizing immunologic methods, to locate the sites within fibrin that are in contact with the platelet membrane (contact peptides). The detailed molecular events involved in the macromolecular interaction will be studied by studying the isolated contact peptides.